Certain compounds in the exhaust stream of a combustion process, such as the exhaust stream from an internal combustion engine, are undesirable in that they must be controlled in order to meet government emissions regulations. Among the regulated compounds are hydrocarbons, soot particulates, and nitrogen oxide compounds (NOx). There are a wide variety of combustion processes producing these emissions, for instance, coal-or oil-fired furnaces, reciprocating internal combustion engines (including gasoline spark ignition and diesel engines), gas turbine engines, and so on. In each of these combustion processes, control measures to prevent or diminish atmospheric emissions of these emissions are needed.
Industry has devoted considerable effort to reducing regulated emissions from the exhaust streams of combustion processes. In particular, it is now usual in the automotive industry to place a catalytic converter in the exhaust system of gasoline spark ignition engines to remove undesirable emissions from the exhaust by chemical treatment. Typically, a “three-way” catalyst system of platinum, palladium, and rhodium metals dispersed on an oxide support is used to oxidize carbon monoxide and hydrocarbons to water and carbon dioxide and to reduce nitrogen oxides to nitrogen The catalyst system is applied to a ceramic substrate such as beads, pellets, or a monolith. When used, beads are usually porous, ceramic spheres having the catalyst metals impregnated in an outer shell. The beads or pellets are of a suitable size and number in the catalytic converter in order to place an aggregate surface area in contact with the exhaust stream that is sufficient to treat the compounds of interest. When a monolith is used, it is usually a cordierite honeycomb monolith and may be pre-coated with gamma-alumina and other specialty oxide materials to provide a durable, high surface area support phase for catalyst deposition. The honeycomb shape, used with the parallel channels running in the direction of the flow of the exhaust stream, both increases the surface area exposed to the exhaust stream and allows the exhaust stream to pass through the catalytic converter without creating undue back pressure that would interfere with operation of the engine.
When a spark ignition engine is operating under stoichiometric conditions or nearly stoichiometric conditions with respect to the fuel-air ratio (just enough oxygen to completely combust the fuel, or perhaps up to 0.3% excess oxygen), a “three-way” catalyst has proven satisfactory for reducing emissions. Unburned fuel (hydrocarbons) and oxygen are consumed in the catalytic converter, and the relatively small amount of excess oxygen does not interfere with the intended operation of the conventional catalyst system.
However, it is desirable to operate the engine at times under lean burn conditions, with excess air, in order to improve fuel economy. Under lean burn conditions, conventional catalytic devices are not very effective for treating the NOx in the resulting oxygen-rich exhaust stream.
The exhaust stream from a diesel engine also has a substantial oxygen content, from perhaps about 2-18% oxygen, and, in addition, contains a significant amount of particulate emissions. The particulate emissions, or soot, are thought to be primarily carbonaceous particles. It is also believed that other combustion processes result in emissions that are difficult or expensive to control because of, for instance, dilute concentrations of the compounds to be removed from the effluent stream or poor conversion of the compounds using conventional means.
In spite of efforts over the last decade to develop an effective means for reducing NOx to nitrogen under oxidizing conditions in a spark ignition gasoline engine or in an diesel engine, the need for improved conversion effectiveness has remained unsatisfied. Moreover, there is a continuing need for improved effectiveness in treating emissions from any combustion process, particularly for treating the soot particulate emissions from diesel engines.
An alternative way to treat the hydrocarbon, particulate, or NOx emissions in an exhaust or effluent stream would be to destroy such emissions using a non-thermal plasma. Plasma is regarded as the fourth state of matter (ionized state of matter). Unlike thermal plasmas, non-thermal plasmas (NTPs) are in gaseous media at near-ambient temperature and pressure but have electron mean energies considerably higher than other gaseous species in the ambient environment. NTP species include electrically neutral gas molecules, charged particles in the form of positive ions, negative ions, free radicals and electrons, and quanta of electromagnetic radiation (photons). These NTP species are highly reactive and can convert hazardous gases to non-hazardous or less hazardous and easily managed compounds through various chemical reaction mechanisms. In contrast to thermal processes (such as thermal plasma), an NTP process directs electrical energy to induce favorable gas chemical reactions, rather than using the energy to heat the gas. Therefore, NTP is much more energy-efficient than thermal plasma.
NTPs call be generated by electric discharge in the gas or injection of electrons into the gas by an electron beam. Electron beams must be accelerated under a high vacuum and then transferred through special windows to the reaction site. The reaction site must be sized with respect to the penetration depth of the electrons. It is much more difficult to scale-up the size of an electron beam reactor than an electric discharge reactor. Therefore, electron beam reactors are less favored than electric discharge reactors.
Among the various types of electric discharge reactors, pulse corona and dielectric barrier (silent) discharge reactors are very popular for their effectiveness and efficiency. However, pulse corona reactors have the major disadvantage of requiring special pulsed power supplies to initiate and terminate the pulsed corona. Consequently, dielectric barrier discharge has become a fast growing technology for pollution control.
Cylindrical and planar reactors are two common configurations for dielectric barrier discharge reactors. Both of these configurations are characterized by the presence of one or more insulating layers in a current path between two metal electrodes, in addition to the discharge space. Other dielectric barrier discharge reactors include packed-bed discharge reactors, glow discharge reactors, and surface discharge reactors.
A variety of known dielectric barrier discharge NTP reactor designs are based upon the use of one or more structural dielectric ceramic pieces coated with a conductive material arranged to form the dielectric barrier-conductor-dielectric barrier configurations. Problematically, structural ceramic substrates provide relatively poor dimensional control with respect to thickness and camber. For example, dimensional thickness and camber of ceramic substrates may vary by, for example, about +/−10% and +/−0.4%, respectively, resulting in variations in dielectric barrier thickness and gaps. This dimensional variation limits the practical operating range for the non-thermal plasma reactor in applications such as after-treatment of diesel exhaust emissions.
Further, structural ceramics comprise a significant portion of the cost factor for current NTP reactor designs based on structural ceramics having dual support and operational dielectric barrier function. In addition, ceramic materials typically used for such applications, including cordierite, mullite, and alumina, have mid-level dielectric constants, limiting the ability to reduce the overall size of the NTP reactor.
Commonly assigned copending U.S. Patent Application Ser. No. 09/812,071 entitled “Non-Thermal Plasma Reactor And Method—Structural Conductor”, which is hereby incorporated by reference herein in its entirety, provides double, single, or null dielectric barrier non-thermal plasma reactor structural conductor elements comprising a structural base conductor. Mechanical strength and durability are provided from one or multiple layers of structural base conductor. For double dielectric structural conductor elements, die base conductor is coated with a high-k dielectric barrier. For null dielectric structural conductor elements, the base conductor is uncoated. For single dielectric structural conductor elements, each exhaust passage has one side coated with high-k dielectric and the other side uncoated.
Uncoated structural conductor reactor elements are most simple to fabricate. However, these require the use of ultra fast (nano-second scale) switching power supplies that are not economical to produce. More typically, the base structural conductor layers are coated with a high-k dielectric so more economical power supplies may be used. It is difficult to achieve a defect-free, durable layer of high-k dielectric over a structural metallic layer for applications having a wide variation in operating temperature, such as for automotive exhaust applications.
What is needed in the art is an improved high capacitance NTP reactor element. What is further needed in the art is an improved high capacitance NTP reactor element that can be manufactured cost effectively while meeting application performance and durability requirements